1. Field of the Invention
The present invention relates to an automatic cut sheet feeding apparatus for feeding stacked sheets one by one into a printer, and particularly to an automatic sheet feeding apparatus used for a printer for an impact printing onto a sheet positioned on a platen by wire dots, shuttle, or other methods.
2. Description of the Prior Art
A printer such as disclosed in U.S. Pat. No. 4,687,362 for example has a sheet fed from a unit 15 is pinched between a platen 10 and pinch rollers 17 of a printer unit 19, fed to be rolled around the platen 10, and the sheet surface printed by impact by means of a wire dot type printer head.
In the state that the sheet is fed along the platen 10, the back of the sheet is pressed against a separating roller 7 by means of tow supply stocks via a V-shaped support 21.
It is now demanded to provide a high quality printing which provides letters with clear edges and easily readable. To provide high quality letters, a platen may be made harder but materials with high hardness (metal, hard rubber, etc.) have low coefficient of friction, inferior sheet feeding capacity, and lowered coefficient of friction due to hardening of its surface and planishing caused by impact with the wire dots, thus induces disadvantages including lowered sheet feeding with the platen. Therefore, U. S. Pat. No. 4,687,362 has enhanced the hardness of the platen to provide better printing quality, inducing a drawback of resistance at the back side of sheets at the stacker section.
Specifically, in the state that the leading edge of a cut sheet which is fed to the printer from the automatic sheet feeding apparatus is grasped by the first sheet feed roller, the end part of that sheet remains being pressed by the sheet feeding roller at the stacker section. Therefore, the feeding operation by the sheet feeding roller at the printer section rotates the sheet feeding roller in the stacker section accordingly, but the friction resistance of the sheet feeding roller bearing and that of sheets due to pressure toward the feeding roller apply loads onto the sheet feeding roller in the printer side, inducing a defect in sheet feeding.
Therefore, in the prior apparatus, the sheet feeding stacker was provided with an electromagnetic plunger which moved the pressure plate backward. Magnetizing this electromagnetic plunger by applying an electric current thereto moved the pressure plate backward to separate the sheet from the sheet feeding roller, thereby removing the sheet feeding load from the sheet feeding roller on the paper feeding unit.
The above existing automatic sheet feeding apparatus supplies a sheet from the sheet feeding stacker to the printer unit by means of the sheet feeding roller, holds the leading edge of the sheet by the sheet feeding roller, moves back the pressure plate to separate the sheet from the sheet feeding roller, and removes a sheet feeding load on the sheet feeding unit with respect to the sheet feeding roller, thereby stabilizing the sheet feeding in the printer.
But, the method moving the pressure plate back by means of the electromagnetic plunger needs to-hold the electromagnetic plunger in the condition that electricity applied until the sheet fed from the sheet feeding stacker is completely sent to the printer together with the printing operation of the printer. Therefore, there were drawbacks that the electricity supplying time to the electromagnetic plunger became long and heated the electromagnetic plunger, a heat resistant electromagnetic plunger was required, mechanism for moving the pressure plate backward became expensive and large, and the sheet feeding unit must be made heavy.
Conventionally, this type of automatic sheet feeding apparatus includes a type that detects the stacked volume of sheets accommodated in the paper feeding stacker by using a sensor, controls the electrical actuator by the detected signal to retract the pressure plate according to the stacked sheet volume, and separates the stacked sheets from the sheet feeding roller, or a type that regardless of the sheet volume accommodated in the sheet feeding stacker, the pressure plate is retracted to a prescribed position by the electromagnetic plunger or the like.
The above former type needs a sensor and an electric powered actuator for operating the pressure plate, decision of the distance the pressure plate is moved backward according to the sheet volume, and a calculation control circuit to control the electric actuator, making the retracting drive mechanism expensive and the electric actuator must be provided additionally, and the sheet feeding unit becomes heavy.
In the above latter type, regardless of the sheet volume accommodated in the sheet supplying stacker, the pressure plate is retracted to the prescribed position, so that when the sheet volume is not much, the top surface of the stacked sheets on the pressure plate and the sheet feeding plate are far separated, and the pressure plate moves backward more than required inducing a loss of time, making the sheets stacked on the sheet supplying stacker are not kept in order resulting from the forward and backward movements of the pressure plate, causing skew, disordered printing position, and greater gap from the sheet feeding roller. Thus, sheets are buckled or not fed smoothly.
In such automatic sheet supply apparatuses, the pressure plate is moved backward by means of an electric plunger so that the gap between the sheet feeding roller and the stacked sheet is kept at a certain level. And sheets exceeding the optimum volume are accommodated in the sheet feeding stacker and when the length obtained by adding the retracted distance of the pressure plate to the stacked thickness of sheets exceeds the maximum retraction of the pressure plate, the electric plunger moves the pressure plate backward and the pressure plate reaches the maximum retraction prior to moving back the prescribed distance. Therefore, the electric plunger is stopped on its way and held with the electricity applied, resulting in over-heating the electric plunger and damaging.
It has heretofore been known is a flat type printer which is for example disclosed in Japanese Utility Model Application Laid-open No. 63-66241.
This type of printer holds the platen disposed to oppose the print head in a fixed state and makes the contact surface of the platen flat against the print head, and a pair of feeding rollers are vertically disposed at the front and back of the platen. These feeding rollers carry the sheet each fed from the automatic sheet feeding apparatus onto the platen in a flat state, and the sheet is printed on the flat surface of the platen.
In the above flat type printer, the platen has a flat area which contacts the print head, making the periphery of letters clear and does not allow the sheet rolled around the platen as in using the roll type platen but has disadvantages that the pair of sheet feeding rollers disposed at the front and back of the platen are made of synthetic rubber or other elastic materials. But since the top and bottom rollers contact very lightly, their sheet feeding force is small, so that when any resistance is applied to the sheet, the sheet slips and is not fed stable, resulting in ununiform line spaces between printed lines on the sheet.
To remedy the above slip trouble, increasing the contact pressure between the top and bottom rollers is considered, but it may result in increasing the roller driving power and requiring a motor with power greater than for driving a tangential contact type roller, and also materials which are not easily deformed are required for the roller supporting shaft and its support members. Thus, other problems are induced including the increase of weight, size, and costs.
And, the flat type printer, as described above, each pair of sheet feeding rollers disposed at the front and back of the platen are made of synthetic rubber or other elastic materials and the top and bottom rollers are contacted lightly, so that the sheet feeding force is small. Therefore, when the sheet feeding roller in the printer starts feeding the sheet carried from the automatic paper feeding apparatus with the operation of the printer, if the bottom end of the sheet is pressed by the pressure plate against the sheet feeding roller on the sheet feeding stacker side, the friction resistances on the sheet feeding roller bearing section and between sheets may load on the sheet feeding roller at the printer side, making the sheet feeding by the sheet feeding roller instable, and making the printed letters' line spaces ununiform on the sheet.
Further, DE3442915A is known disclosing a sheet feeding mechanism to feed sheets one at a time in an office printing apparatus.
This sheet feeding mechanism is provided with a control mechanism for the printing mechanism to rotate a separating roller via a drive mechanism and to move the sheet stuck back into the drive jointing mechanism.
This sheet feeding mechanism needs a gear (forming a part of the drive mechanism) with a peripheral length corresponding to the sheet length or cam (forming a part of the control mechanism). Therefore, to print a long-size sheet, the gear size must be larger. And, when the distance between the magazine and the printing mechanism is large, the sector's (forming a part of the drive mechanism) radii must be increased, and the above gear or cam diameter must be made large, thus the sheet feeding apparatus cannot be made small. Further, when the gear or cam diameter is determined to conform to a long-sized sheet, a time of 1 cycle is same with when a short-sized one is used, and this is very inefficient to print a large volume. For example, when the switch is turned off because a sheet clogs with the tappet (forming a part of the control mechanism) having the raised part of the above cam on its back end (the state that the supporting part retracts, and the separating roller and the stacked sheet are separated), to resume printing it is necessary to return the above tappet and the cam to the normal start position, which is a troublesome work. If they are not returned, the sheet is not fed smoothly, causing inferior printing.